This disclosure relates to a vaporized fuel processing apparatuses having an adsorption chamber, a tank port, a purge port, an atmospheric port and a heater. The adsorption chamber is filled with an adsorbent capable of adsorbing and desorbing fuel vapor vaporized in a fuel tank. The tank port is communicated with the tank port. The purge port is configured to discharge the fuel vapor, which has been desorbed from the adsorbent, to the outside of the adsorption chamber. The atmospheric port is open to the atmosphere. The heater is disposed between the adsorption chamber and the atmospheric port.
The vaporized fuel processing apparatus, which is also referred to as “canister”, is mounted on a vehicle such as automobile in order to prevent leakage of fuel vapor, which has been vaporized in a fuel tank, to the outside of the vehicle. In detail, the fuel vapor, which has been vaporized in the fuel tank, flows into the adsorption chamber via the tank port and is selectively adsorbed into the adsorbent disposed in the adsorption chamber. However, the adsorbent has an adsorption capacity for the fuel vapor and cannot adsorb the fuel vapor over this adsorption capacity. Thus, it is necessary to periodically desorb the fuel vapor from the adsorbent in order to recover adsorption ability of the adsorbent. Accordingly, an atmospheric air is introduced into the adsorption chamber via the atmospheric port as purge air due to negative pressure in an intake pipe connected to an internal combustion engine and the like in order to desorb the fuel vapor from the adsorbent. The desorbed fuel vapor is discharged to the outside of the adsorption chamber via the purge port.
The adsorbent has a characteristic that the higher the temperature is, the lower the adsorption capacity for the fuel vapor is, and that the lower the temperature is, the higher the adsorption capacity for the fuel vapor is. Thus, when desorbing the fuel vapor from the adsorbent, the higher the temperature is, the larger the desorption amount of the fuel vapor is, and the lower the temperature is, the smaller the desorption amount of the fuel vapor is. Accordingly, when desorbing the fuel vapor from the adsorbent, it is preferable that the temperature is as high as possible in order to improve desorbing efficiency (recovery efficiency of the adsorbent). However, when desorbing the fuel vapor from the adsorbent, the temperature of the adsorbent tends to decrease due to heat of vaporization of the fuel vapor. Thus, the desorbing efficiency can be improved by providing a heater at the upstream of the adsorption chamber and heating the purge air.
Japanese Laid-Open Patent Publication No. 2012-102722 discloses a vaporized fuel processing apparatus having a heater for heating purge air. The heater has a heating element, which generates heat by electricity supply, and a fin heat exchanger, which is joined to the heating element and extends from the heating element both to the tank port side and to the adsorption chamber side. With respect to the heater, the heating element is positioned at a center of the fin heat exchanger with respect to a flowing direction of the purge air. The fin heat exchanger has a plurality of fins arranged in parallel to each other at regular intervals.
In the vaporized fuel processing apparatus of Japanese Laid-Open Patent Publication No. 2012-102722, a diffusion plate having a plurality of diffusion holes is provided between the heater and the atmospheric port in order to radially diffuse the purge air introduced from the atmospheric port and to uniformly supply the purge air to the entire heater. The diffusion holes of the diffusion plate are arranged such that the opening area of the diffusion holes at the center area just below the atmospheric port is the smallest, and such that the opening area of the diffusion holes gradually increases from the center area toward a circumferential edge of the diffusion plate.
The purge air is introduced from the atmospheric port via the heater into the adsorption chamber. Thus, with respect to the flowing direction of the purge air, heat exchange efficiency upstream of the heating element is lower than heat exchange efficiency downstream of the heating element. That is, the heat exchange efficiency by a part of the fin heat exchanger, which extends from the heating element to the atmospheric port side, is lower than the heat exchange efficiency by another part of the fin heat exchanger, which extends from the heating element to the adsorption chamber side. Accordingly, at the upstream of the heating element, the fin heat exchanger cannot exert its maximum performance. In the case of the vaporized fuel processing apparatus disclosed in Japanese Laid-Open Patent Publication No. 2012-102722, because the heating element is positioned at the center of the fin heat exchangers with respect to the flowing direction of the purge air, heating of the purge air by the heater is inefficient. Further, this decreases the space efficiency for the fin heat exchangers.
Sometimes, the canister is horizontally disposed such that a flow passage for gas within the adsorption chamber extends horizontally. In this case, because the specific gravity of the fuel vapor is heavier than that of air, the adsorption amount of the fuel vapor at a lower area within the adsorption chamber tends to be large. Thus, when the canister is disposed horizontally, it is preferable that the heating efficiency of the purge air by the heater increases toward the bottom. In the case of the vaporized fuel processing apparatus disclosed in Japanese Laid-Open Patent Publication No. 2012-102722, the fins of the fin heat exchanger are arranged at regular intervals, so that it would be difficult to preferentially heat a lower area within the adsorption chamber.
Sometimes, the atmospheric port is formed at a position eccentric relative to the center of the adsorption chamber in the radial direction at an end of the adsorption chamber facing the atmospheric port. In the case of the diffusion plate disclosed in Japanese Laid-Open Patent Publication No. 2012-102722, because the opening area at the center is the smallest, the position of a portion having the smallest opening area of the diffusion plate is deviated from the position of the atmospheric port in the radial direction. This cannot uniformly supply the purge air to the heater, so that the desorbing efficiency is low.
When the canister traps the fuel vapor generated in the fuel tank, gas flows within the adsorption chamber from the tank port toward the atmospheric port. Thus, the fuel vapor concentration in the gas flowing through the adsorption chamber decreases from the tank port toward the atmospheric port. Accordingly, the adsorbing efficiency for the fuel vapor decreases toward the atmospheric port. In the case of the vaporized fuel processing apparatus disclosed in Japanese Laid-Open Patent Publication No. 2012-102722, the adsorption chamber is divided into a plurality of compartments, and the compartments are filled with the same adsorption material. Therefore, there has been a need for improved vaporized fuel processing apparatuses.